Toroid transformer electrostatic shield



Oct. 15, 1968 R. H. RUSSELL ET Al. 3,406,364

TOROID TRANSFORMER ELECTROSTATIC SHIELD Filed March l, 1965 ra/Mw :Hoy/ca,

Umm/A- (1D/z. BERT;

United States Patent O 3,406,364 TOROID TRANSFORMER ELECTROSTATIC SHIELD `Robert H. Russell, Altadena, and Donald Boyko and John A. Gilbert, Pasadena, Calif., assignors to Electro- Optical Systems, Inc., Pasadena, Calif., a corporation of California Filed Mar. 1, 1965, Ser. No. 436,123 8 Claims. (Cl. 336-69) This invention relates to toroid transformers and more particularly to a novel interwinding shield for eliminating the electrostatic coupling between primary and secondary windings of small toroid transformers.

Recent developments in materials in the prior art have made it possible to provide electrostatic shielding for small torroid transformers. Under the usual conditions f use of such transformers any shielding between windings would increase the interwinding and winding-to-core capacitances because of the small dimensions involved.

In the prior art a transformer as small as 3%4 in diameter could be shielded only with great diiculty. The most recent technique above referred to consisted of the steps of painting a conductive coating on the toroid core to provide the shielding surfaces, following this with a layer of insulative material for several alternate layers of the shield and insulation upon the core. To achieve effective shielding by this prior art technique many layers of conductive paint and insulative material had to be used as above described. The result was accomplished at a substantial cost in time and materials and with less than a desirable degree of reliability. The increase in rate of failure was as high as five times as great as for an unshielded toroid transformer.

The present invention contemplates a novel means whereby the electrostatic shielding between the windings of a small toroid transformer may be achieved without any of the attendant diiculties which were characteristic of the only other known prior art method. No conductive paint and insulative materials in alternate layers intervene between the winding and core. The new technique according to this invention can be implemented with little mechanical diiculty in inserting the shield.

According to the present invention the windings of the toroid are made very close to the core. The core is wound about, but insulated from, a toroidal conductive steel bobbin. In the absence of a ground reference, capacitance between each winding and the core may be considered as series capacitors and the capacitances between primary and secondary windings in parallel with the series capacitances. Shielding material is added across the toroid, perpendiculairly to the plane of the toroid, and is connected to the conductive toroid bobbin electrically. The result is an electrostatic shield between primary and secondary windings, which are wound about the toroid on either side of the shielding material, and due to the shield and its connection to the bobbin, remaining coupling capacitance between the primary and secondary windings is reduced to a minimum.

EElectrostatic isolation achieved with the present invention is the result of using the conductive bobbin as a direct interwinding shield with the electrostatic shield insert connected electrically at a single point to the toroid bobbin. The effect of inserting the shield is to provide a ground reference return path between the windings which inhibits the effects of potential changes on the primary electrostatically inducing potential changes in the secondary.

'ing means which is both reliable and The electrostatic shield may be utilized as a low impedance dissipation path for noise currents.

Fabrication of toroid transformers according to the present invention is simplified as compared to the prior art transformers because windings can now be made continuously and without the danger of injury to layers in the process of applying insulative paints or coatings to the windings. Windings are accessible and not obscured by subsequently applied layers as in the prior art. There is no mechanical interference between windings, as in prior art toroid transformers, when the electrostaticy shield is added.

The toroid transformers, implemented in accordance with this invention, are particularly useful in direct-current-to-direct-current converters where electrostatic shielding is 'essential for interference-free and low-noise operation.

Accordingly it is an object of this invention to provide a novel improved and more reliable toroid transformer with electrostatic shielding between the primary and secondary windings thereof for low-noise and interferencefree operation in electric circuits.

It is another object of this invention to provide in toroid transformers of small size, an effective electrostatic shieldpresents no mechanical interference between windings.

It is further an object of this invention to provide a novel electrostatic shield for a toroid transformer wherein the shield is across the toroid and electrically connected to one point therein, the primary and secondary windings of the transformer being respectively wound on either side of the shield insert about the toroid.

It is still another object of this invention to provide in a toroid transformer a conductive toroid with an electrostatic shield inserted across the toroid, the shield being electrically in Contact with said toroid at one point thereon.

It is a still further object of this invention to provide in a toroid transformer an effective electrostatic shield between primary and secondary windings including a thin toroid bobbin of conductive material with a wrapped ferro-magnetic core upon which is wound insulative tape completing less than a full circumferential turn of the bobbin, and an electrostatic shield inserted perpendicularly to the -plane of the bobbin across the bobbin and in contact with the conductive material thereof at the point of incompletion of said turns of insulative core wrapping material.

These and other objects of this invention will become more clear from Vthe specification which follows in which a preferred embodiment of the invention is disclosed. The embodiment shown and described should be taken as being merely illustrative of the invention and not limited to the embodiment described since those skilled in the arts appertaining hereto will be able to devise other embodiments in the light of the teachings herein when taken together with the drawings and the appended claims.

In the drawings:

FIGURE 1 is a representation of the relationship between the windings of a transformer and the core with respect to the capacitances between them;

FIGURE 2 shows the relationship of capacitances when this invention is employed;

FIGURE 3 is a perspective drawing of a' toroid transforrner in accordance with the present invention;

FIGURE 4 is a cross-section through 4 4 of FIGURE 3; and

FIGURE 5 is a cross-section through 5-5 of IFIG- URE 3.

In (b) of FIGURE 1 the| electrical circuit equivalent of the capacitive relationships above-described'i'sshown and can be seen tore'present a1 pairfof capacitors G1 and C2 in series, lwith .a capacitor C3 connected -across the seriespair asa parallel element therewithTh`e resulting capacity Ct to be t C1-PC2403 between primaryfand secondary canV beseen The schematic representation in FIGURE 2 shows a shield according to this invention interposed between primary winding 10 and secondary winding 11 changes the capacitance conliguration. Now the interwinding capacitance can be represented by two capacitances, each 2C3. Each capacitor, 2C3, is between a winding and shield 13. Shield 13 is in the form of a T-shapcd plate which is more fully'described hereinbelow, and which is inserted in the core of a toroid transformer as further described below .for the purpose of intersectingthedirect stray coupling capacitances between primary and secondary.

When shield plate 13 is connected to a low impedance ground return or zero reference path :any electrostatic field from either primary or secondary winding generates. a charge movement only in said shield plate, and not across to the other winding, thereby eliminating first order electrostatic coupling between windings. That is, the interwinding capacitance is eliminated. In a typical case the reduction in the interwinding capacitance achieved was from 12 pf. to .75 pf. t.

The physical structure of a transformer 'according to this invention is shown in FIGURE 3 in a Iperspective view. Sections taken through 4-4 of FIGURE.3 and 5 5 of FIGURE 3 are shown in FIGURES'4 and Sirespectively. Inreferring to `FIGURES 3-5 the reference characters u'sed correspond with those in .the other figures and are consistent in that like items employ like reference characters in each figure. 2 L v 'i w In FIGURES 3-5 a lbobbin 12 can be seen to have .a spool-like shape. Bobbin12 is of a conductive material-and preferably ,of a conductive origin such as steel or copper and the like. A flat thin tape or ribbon of insulative material 21'coats or covers the entire inside of bobbin 12 including the inner surfaces of the bobbin flanges. Then about the bobbin 12 a ferromagnetic ribbon 19 is wound circumferentially until bobbin 12 is filled. Thus ribbon or tape 19 forms a core with shielding provided by the spool- 20 form a toroidal tran'sformer core 25. On an axis 90:o 70

removed in one direction from the gap 14 a primary winding 10 is placed on core 25; and opposite a secondary winding 11 is wound. c v L A T-shaped electrostatic metallic shield 13 is inserted Shield 13 has a bent over tab 23 on an end thereof which inthe central aperture 22 of'core 25, as indicated at 1'7. I:

12 as shown at 15, in the gap area 14-16 as previously described.

The resulting toroid transformer configuration can e tlc taplrflllng periphery. Wori'rli]aboutthe' resplntingltoroid core 25 is an insulative tape 20 which does not completely cover the bobbinf=12 'but-leavesv exposednanzareaful'fl; to'vwhichjelctricalllya'conductive contact` .can 'be-Jmade 4asfalt-115. -Af-T- shaped` sliield-13/ 17 is insertedintheftoroid wellat.22 and a tab 231thereo'n is welded at=15 to thezexposedrporf tion- 16fof-bobbin12.,A primary winding 10 and Iaseco'ndaryrwihding: llfareewoundY about' cor-e 25-respectivelyon opposite 5 :sides fof: shield'- 13./.17-. Thel Ymaterial .of shield 13/ 17 is conductive,=such=.as='c0pper, brass, onalurninum, which-willpreventstr-ay ,capacitive coupling of-electro-f static. chargesMacross*fromV primary-f1.0 to secondary :11: Since the 'pertinent 'magnetic 'coupling is` through'the'cor'e 25, the shield 13/17 does-inet affectanyf'tbub the 'stray coupling; f l The shield 1'3/175- being` positioned betweenK primary 10 and secondary 11, and electrically tied vto the conductive bobbin 12,isl -grounded in use" as indicated at '-14 in FIG- URI-3 2, sothatfshield 137;17 is, in fact, atvground return reference yand'provides'a low impedance path for any electrostatic charge generated by primary` winding '10 orI secondary 11 preventing thetransfer of electrostatic noise or interference-pulses' between lprimary 10 and secondary 1l. y .'What is claimed isz 1 1. In atoroid transformer, the combination lofz r a torodialbobbin ofconductive material;

a ferromagnetic tape wrapped' circumferentially about and insulated from saidbobbin 4of conductive material; fr: H a -insulation 'material-wrapped upon said -bobbin, leaving only a small space of said conductive-materiaof said bobbin exposed; anelectrostatic shield plate inserted across the: diameter fof 'saidibobbi'ng making electrical and mechanical contact with-said conductive material lof said 'toroidal bobbin only at said gexposed'sm'allfspace, said vshield being in arplane perpendicular .tofthe plane ofthe circumference of said bobbinsaid bobbin, said ferrof magneti'ctape, and. said'v insulating; .material forming -aninsulated-toroidal' core withv said shield inserted f-thereinj" vL' "Ha primarywinding wound on :said core von'one side of said shield; and 'a secondary windingfwound von -said coreror'r'the 'other fside of saidv shieldopposite` said :primary winding 5^ L hereby,-.the4 interwindingcapacitan'cesf between said primary. winding and@ 'said secondary -windin'g are .sa minimum" andi said? shield- 'thereby provides 'a' maxi',-A Amum.interwinding'electrostatic shielding against elecl .-'trostaticallyfinduced-potentials:betweentsaidwwindings to reduce interferenceand' n'oiset that lmay:bel-other.- wise generatedlvin'Ithe-use Of-said'-transforniera2.=f'-Animproved .toroid transformer, comprisingt '-:a conductive-toroidal 'bobbin fhaving'. a kcentral aperture; a ferromagnetic tape "insulativelywrapped"upon said bobbingjw u. L insulation material Wrapped fabutf'saidbobb'in,'1with l said#ferromagnetictape thereon, leaving onlyafsniall space of said conductive material fof* s'aidf"bo'bbii1 .'rexposedgam 'n 'L -anelectrostatic'shieldfplate insel' din'1saidfcentral aperturel acrossthe diameter 'of @saidbob'bin,-`andffadapted tozmakelectricalicontactwith S,aid"conductivel bobbin enljyleat rsaidexposedarsmall fspa'c, fs'aid'shieldbeing in aplane penpendicular-fto'theiciicunfefrentia plane of? said" bobbin, sidhobbin, Said 'ferromagnetictapel id saidhinsulation. material fornring'ffari insulated toroidal core with said shield inserted thereinff^f x A w.

.. a primary winding wound on said core on one side V static shielding against electrostatically induced potentials between said windings to reduce interference 'l and noise that may be otherwise generatediin the' use of said transformer.

3. In a toroid transformer;l the combination ofi a toroidal bobbin of conductive material;

a ferromagnetic tape wrapped circumferentially about and insulated from said bobbin of conductive material,

insulation material wrapped upon said ferromagnetic tape and said conductive bobbin leaving only a small space of said conductive bobbin exposed;

an electrostatic shield plate inserted across and perpendicular to the diameter of said bobbin, in conductive contact therewith;

said bobbin, said ferromagnetic tape, and said insulation material forming an insulated toroidal core with said shield inserted therein;

a primary coil wound on said core on one side of said shield; and a secondary coil wound on said core on the other side of said shield opposite said primary coil;

whereby, the interwinding capacitances between said primary winding and said secondary winding are -minimized and said shield provides shielding against electrostatically induced potentials between said windings to reduce interference and noise that may be otherwise generated in the use of said transformer.

4. In a toroid transformer, the combination of:

a toroidal bobbin of conductive material;

a ferromagnetic tape wrapped circumferentially about and insulated from said bobbin of conductive material;

insulating material surrounding said ferromagnetic tape and said conductive bobbin leaving only a small space of said conductive bobbin exposed, said tape and said bobbin comprising an insulated toroidal transformer core;

an electrostatic shield plate inserted across the diameter of said core in electrical contact with the conductive material of said bobbin;

a primary Winding on said core on one side of said shield plate; and

a secondary winding on said core on the other side of said shield plate,

whereby the interwinding capacity is minimized and electrostatic coupling between said primary winding and said secondary winding eliminated thereby to shield said windings from electrostatically induced charges from 4one of said windings to the other, and reduce interference or noise voltages that may otherwise be developed in said windings in the use of said transformer.

5. In a toroid transformer the combination of:

a toroidal core comprising a bobbin of conductive material having a ferromagnetic tape wrapped circumferentially about and insulated from said bobbin of conductive material;

insulating material surrounding said ferromagnetic tape and said conductive bobbin having only a small space of said conductive bobbin exposed;

an electrostatic shield inserted perpendicularly to the plane of said core across the diameter of said core making electrical and mechanical contact at said exposed small space with said conductive material of said bobbin;

a primary winding on said toroidal core over said ferro- Y magnetic tape wrapping on one side of said shield; and,."f

a secondary'winding wound on said toroidal core over said ferromagnetic tape wrapping on the other side of said shield diametrally opposite said primary winding, i t f 'i whereby the interwinding capacitances between said primary winding and said secondary winding are reduced, and said electrostatic shield provides a maximum interwindingelectrstatic shielding to reduce interference and noisethat may otherwise bergenerated by said transformer in use thereof.A

6 A toroidtransformer. comprisingthe combination of:

a toroidal core formed by a bobbin of conductive material wrapped circumferentially with a ferromagnetic tape;

insulating material surrounding said ferromagnetic tape and said conductive bobbin leaving only a small space of said conductive bobbin exposed;

an electrostatic shield inserted perpendicularly to the plane of said core across the diameter of said core in electrical contact with said conductive material at said exposed small space;

a primary winding on said toroidal core on one side of said shield; and,

a secondary winding on said toroidal core .on the other side of said shield diametrally opposite said primary winding,

whereby the interwinding capacitances between said winding is reduced by said electrostatic shield to thereby provide a maximum interwinding electrostatic shielding for reducing interference and noise that may otherwise be generated by said transformer in use thereof.

7. A toroid transformer comprising the combination of:

a toroidal ring .of conductive material wrapped circumferentially about with a ferromagnetic tape, said tape insulated from said conductive ring;

insulating material surrounding said ferromagnetic tape and conductive ring leaving only a small space exposed;

an electrostatic shield inserted across the diameter of said ring in electrical contact with said conductive material at said exposed small space;

said shield being in a plane perpendicular to the plane of said ring;

l a primary winding wound on said toroidal ring over said ferromagnetic tape on one side of said shield;

a secondary winding wound on said toroidal ring over said ferromagnetic tape on the other side of said shield diametrally opposite said primary winding; and

whereby the interwinding capacitances between said primary winding and said secondary winding is minimal and said shield thereby provides a maximum interwinding electrostatic shielding to reduce interference and noise that may otherwise be generated by said transformer in use thereof.

8. In a toroid transformer the combination of:

a toroidal ring of conductive material;

a ferromagnetic tape wrapped circumferentially about said conductive toroidal ring; insulation material wrapped upon said ferromagnetic tape and said conductive ring leaving only a small space of said conductive material of said toroidal ring exposed;

an electrostatic shield inserted across the diameter of said ring making electrical and mechanical contact at said exposed small space with said conductive material of said toroidal ring; said shield being in a plane perpendicular to the plane of said ring;

a primary winding insulatively Wound on said toroidal ring over said ferromagnetic tape wrapping on one side .of said shield; and,

-a secondary Vwinding wound insulatively on said toroidal ring over said ferromagnetic tape wrapping on the other side of said shield diametrally opposite said primary :windingg v whereby the capacitances representedfby saidfprimary lWi nding,to1said. toroidalring; said. secondary :winding totsaid toroidal ring,y said primary winding-to said v electrostatic: shield and.. saidY secondary.y winding `t0l said electrostatic shield result in` a minimal Ainterwindingrapacit-ancc/randsaid shieldtherebypror, 10

vides-a maximum interwinding electrostatic shielding against electrostatically induced potentialspbetween said/windingto reduceinterference and noise n that may otherwisebe generated by said transformery in use thereof.

References Cited UNITED STATES :PATENTS 

1. IN A TOROID TRANSFORMER, THE COMBINATION OF: A TORODIAL BOBBIN OF CONDUCTIVE MATERIAL; A FERROMAGNETIC TAPE WRAPPED CIRCUMFERENTIALLY ABOUT AND INSULATED FROM SAID BOBBIN OF CONDUCTIVE MATERIAL; INSULATION MATERIAL WRAPPED UPON SAID BOBBIN, LEAVING ONLY A SMALL SPACE OF SAID CONDUCTIVE MATERIAL OF SAID BOBBIN EXPOSED; AN ELECTROSTATIC SHIELD PLATE INSERTED ACROSS THE DIAMETER OF SAID BOBBIN, MAKING ELECTRICAL AND MECHANICAL CONTACT WITH SAID CONDUCTIVE MATERIAL OF SAID TOROIDAL BOBBIN ONLY AT SAID EXPOSED SMALL SPACE, SAID SHIELD BEING IN A PLANE PERPENDICULAR TO THE PLANE OF THE CIRCUMFERENCE OF SAID BOBBIN, SAID BOBBIN, SAID FERROMAGNETIC TAPE, AND SAID INSULATING MATERIAL FORMING AN INSULATED TOROIDAL CORE WITH SAID SHIELD INSERTED THEREIN; A PRIMARY WINDING WOUND ON SAID CORE ON ONE SIDE OF SAID SHIELD; AND A SECONDARY WINDING WOUND ON SAID CORE ON THE OTHER SIDE OF SAID SHIELD OPPOSITE SAID PRIMARY WINDING, WHEREBY, THE INTERWINDING CAPACITANCES BETWEEN SAID PRIMARY WINDING AND SAID SECONDARY WINDING ARE A MINIMUM AND SAID SHIELD THEREBY PROVIDES A MAXIMUM INTERWINDING ELECTROSTATIC SHIELDING AGAINST ELECTROSTATICALLY INDUCED POTENTIALS BETWEEN SAID WINDINGS TO REDUCE INTERFERENCE AND NOISE THAT MAY BE OTHERWISE GENERATED IN THE USE OF SAID TRANSFORMER. 